The present invention relates to a pulse duration modulator and to a low power switching power supply having a pulse duration modulator.
Conventionally, a pulse width or pulse duration modulator in prior art comprises a voltage comparator for producing a ramp wave from an oscillator with a D.C. voltage, and input an analog adjusting means for compensating the device due to the unstable offset voltage of the voltage comparator --to stabilize the starting voltage of the ramp wave.
As mentioned above, the pulse duration modulator requires an oscillating circuit for ramp wave generation, but it is not easy to synchronize this oscillator with the oscillator in the system in which it is used. Therefore, it is difficult to control the pulse wave--the duty ratio--, and the S/N ratio in the system is degraded by the noise generated from the oscillator itself.
Moreover, the conventional circuit used for the analog adjusting means is overly complicated, and it is difficult to decrease current consumption because the voltage comparator current consumption is very high even in a stable state during switching time. Also, special consideration to the IC design of an input differential amplifier in the comparator is required in order to match the characteristics of the transistors.
The object of the invention is to solve the aforementioned problems and therefore to offer a small type switching power source with the desirable characteristics of precise control of pulse duration, low operating voltage, low current consumption and low noise.